This invention relates to a zigzag sewing machine, more particularly to a zigzag sewing machine in which minimum crossfeeding pitch of a feed dog controlled by a crossfeed device is smaller than the minimum laterally moving pitch of a needle and thus the amplitude or relative moving distance between the needle and a fabric can be minutely set.
Generally speaking, prior-art electronic zigzag sewing machines have in an arm a needle bar vertical drive mechanism, a needle bar lateral drive mechanism, and a needle bar lateral drive stepping motor for controlling the needle bar lateral drive mechanism, and have in a bed a vertical feed mechanism for moving a feed dog up and down, a feed mechanism for moving the feed dog back and forth, and a feed stepping motor for driving the feed mechanism. The sewing machines can sew a variety of stitches such as straight stitch, zigzag stitch, buttonhole stitch, overlock stitch, and a plurality of pattern stitches forming characters, ideograms, etc. The maximum lateral moving range of the needle bar is about 7 mm, the maximum width of patterns is also 7 mm, and the minimum lateral moving pitch of the needle bar is about 0.5 mm.
Recently new-type zigzag sewing machines have been proposed. The sewing machines are provided with a crossfeed mechanism for driving a feed dog from side to side by means of a stepping motor or a solenoid in synchronization with the feeding movement of the feed dog. Thus, the sewing machines can sew patterns with larger width.
For example, in U.S. Pat. No. 4,691,654 a zigzag sewing machine is suggested which comprises a vertical feed mechanism for moving a feed dog up and down, a feed mechanism for moving the feed dog back and forth, and a crossfeed mechanism for moving the feed dog from side to side. The vertical feed mechanism uses an eccentric cam fixed on a shaft rotated by a sewing machine motor. In the feed mechanism, the inclination of a feed controller is adjusted by a feed stepping motor, the eccentric cam moves a slider by means of a lever and a link bar, and the feed dog is moved back and forth via a link mechanism comprising a connecting bar and a feed member. In the crossfeed mechanism, a feed dog supporting member is pressed to and contacted with a needle sleeve on its side wall, and a crossfeed stepping motor rotates a lateral drive member moving the feed dog laterally while the feed dog moves back and forth. The center of the needle sleeve is beside the center of rotation of the lateral drive member.
Since the minimum lateral moving pitch of the prior-art zigzag sewing machines is about 0.5 mm, the sewing machines cannot sew either beautiful patterns with curved lines, such as characters and ideograms, or stitches parallel to creases of a fabric. A sewing machine having minimum laterally moving pitch of about 0.1 mm can sew such beautiful patterns and neat stitches, but an expensive stepping motor with high resolution and high-speed responsiveness is required. The above-mentioned zigzag sewing machine of U.S. Pat. No. 4,691,654 has the same problems, because the purpose of its crossfeed mechanism is sewing patterns of larger width than other prior-art sewing machines.